1. Field of the Invention
The invention relates to a radome or cover for protecting a radar device while presenting a decorative metallic image, especially for radar devices disposed behind the front grill of an automobile.
2. Antecedents of the Invention
In general, radio transmitter-receiver devices, such as a millimeter-wave radar, have been employed as sensors for vehicular collision avoidance and adaptive cruise control systems.
In a radar system or the like that measures obstacles in front of an automobile and the distance between automobiles, the antenna was preferably positioned in the center at the front of the vehicle to obtain maximum performance. Although the radar antenna could be installed near the front grill of automobile, it was preferable to conceal the antenna from view due to its non-aesthetic appearance and to shield the antenna from external environmental factors such as weather and airborne contaminants.
In order protect the antenna and avoid radio interference and signal losses of the radar device, it has been proposed to provide a radar window capable of transmitting radio waves in the front grill corresponding where the radar antenna was located. This allowed radio waves to pass in and out through the window. However, the radar window diminished the appearance of the front grill due to the interruption of the pattern of the grill structural elements. Further, unsightly internal portions of the vehicle, like the radar transmitter-receiver, could be seen through the radar window.
In U.S. Pat. No. 6,328,358 unity between the radar window and the front grill body was provided. The radar window as disclosed in U.S. Pat. No. 6,328,358 was formed by laminating a plurality of resin layers formed with concavity and convexity. This component provided an impression by a metal layer deposited with concavity and convexity between the resin layers such that the fin member of the front grill appeared to extend across the radar window without interruption.
Indium was used as a metal deposited in such radar window. When depositing indium on a deposit member, indium was not deposited on the surface in a uniform film manner, but deposited in a nanometric insular manner. In other words, when indium was deposited on the deposit member, the surface of the deposit member comprised a nanometric combination of a deposit portion where indium was deposited in an insular manner and a non deposit portion where nothing was deposited.
In this case, radio waves could pass in and out through the non-deposit portion and the surface of the deposit member could be recognized upon viewing as a member that had metallic luster, since the deposit portion had indium deposited in a nanometric insular manner.
This selective way of deposition complicated the process of application of the indium metals. Further radio waves did not pass in and out satisfactorily when the deposit portions were formed extremely close together. The conductivity of the metals required the use of low density evaporation methods such thermal evaporation. These methods did not guarantee a uniform deposition in thickness throughout the member or between members produced in the same batch. Other methods of deposition would guarantee uniform insular deposition such sputtering, but sputtering provided a metal density that generates a high level of attenuation, making the system useless for a radome application in front of a radar antenna.
U.S. Pat. No. 6,328,358 disclosed a thin metallic layer comprising indium deposited on a metal portion area that could be viewed from the outside at a plastic-plated member for the beam path of a radar device. However, it was necessary to ensure a bright design and the reliability of durability for radio transmittance by forming a stable protective layer so as not to allow the indium bright film layer to undergo exfoliation or be damaged by an external force, or to be corroded by an external environmental stress such as water or polluted air.
This was due to the following: indium is a very soft metallic material with a value of 1.2 in the Mohs hardness scale; indium corrodes under the aforementioned environmental stress since it is basically a metallic material; it is necessary to ensure the reliability of durability by securing the film thickness with certainty such that the bright-effect design of indium can be obtained without thickening the indium film layer more than is necessary, since a radio transmission loss occurs as a conductivity loss based on the fact that indium is basically a metallic material; and the indium layer melts due to the heat of molten resin when successively conducting secondary formation of the lining resin on a resin-molded component in which a film is formed on the surface of a base body in advance, since the melting point of indium is 156° C., which is extremely low, for example.
Although indium film is suitable for a film of an emblem or the like as it shows a metallic color, it poses problems in that it readily experiences exfoliation and lacks durability and abrasion resistance. Also, indium film may corrode, since it is basically a metal. Thus, when a ceramic film comprising silicon dioxide is disposed, the durability is improved and the film, or paint, can be protected. However, the ceramic film comprising silicon dioxide is colorless, so that the appearance of a metallic color, for example, cannot be provided.